1. Field of the Invention
The present invention relates to the field of nuclear medicine camera systems. More specifically, the present invention relates to the field of nuclear medicine camera systems utilizing gated SPECT acquisition techniques,
2. Prior Art
In an attempt to more accurately diagnose Coronary Artery Disease (CAD) and generally diseases of the heart, specialized nuclear medicine camera systems have been developed to provide physicians with vital information regarding the structure of the heart and myocardial wall tissue. The images of the heart structure provided by these non-intrusive nuclear medicine camera systems illustrate tissue and structure that would not be otherwise visible without the application of nuclear medicine or other non-intrusive method. Gamma cameras of the Anger type are well known cameras in the field of nuclear medicine. These cameras receive energy emissions from a radio-pharmaceutical that is introduced into a patient and concentrates or localizes within the organ or tissue of interest for imaging. Such cameras are used extensively in nuclear medicine as radiation detectors for establishing the distribution of the radio-pharmaceutical within the organ or tissue of interest. Such a camera is described in detail in U.S. Pat. No. 3,011,057 issued to Anger in which a typical gamma camera apparatus is disclosed for collecting information after introduction of radionuclides via inhalation, injection or ingestion.
Single Photon Emission Computerized Tomography (SPECT) is a type of nuclear camera imaging system wherein the radiation detector of the camera system is rotated about an organ or tissue (referred as an object of interest) and images of the object of interest are recorded at discrete angles of rotation. By projecting back each image of the object at these different rotation angles, a total image or reconstructed volume can be generated and images may be observed at different slices within the object volume itself. In other words, three dimensional image information can be generated by SPECT camera systems of the object of interest. Typically the camera detector rotates 180 degrees or full 360 degrees around the object (patient) during the image acquisition phase of the SPECT camera system. Once the image data is collected by the camera system, it is processed by a computer system where the tomography is performed and the images may be generated qualitatively and studied on a computer display screen. Such SPECT camera systems are well known in the art of nuclear medicine.
With respect to prior art SPECT camera systems used in non-gated cardiac perfusion studies, heart tissue is studied for infarct areas and ischemic areas by examining the heart under two different conditions, a stress condition and a rest condition. Perfusion refers to the blood flow to the heart in the areas of interest. The radionuclide introduced to the heart will follow the blood flow and thus perfusion is determined by monitoring the resultant radiation from the radionuclide. An infarct area is an area of the heart that is not functional and may be composed of dead tissue. This area may not take up much if not any of the introduced radio-pharmaceutical. An ischemic area of the heart is an area that may perhaps function normally during rest conditions, but will not function normally during cardiac stress conditions. In order to detect an ischemic area, non-gated SPECT systems must collect image data at cardiac rest and stress conditions, requiting two sessions. Therefore, according to the two conditions under study as described above, the ischemic area will be detected by comparing the images of the heart during the stress condition and the rest condition. In this prior art system, SPECT camera systems are used to examine the heart after the heart is subjected to a stress condition, typically by having the patient run a treadmill. The heart imaging session of the prior art camera systems takes approximately 30 minutes. Next, the patient is allowed to rest for at least four hours and a new imaging session is performed on the heart during a rest condition. The physician then compares the results of the imaging at rest and at stress. An ischemic area may show up as a myocardial defect on images of the heart taken during stress conditions but may show up normally on images of the heart taken during rest conditions. An infarct area should show up as a defect in the heart at both rest and stress conditions.
The above prior art non-gated SPECT perfusion method for determining CAD, such as infarct areas and ischemic areas, is not the most advantageous system. This is the case because two imaging sessions must be performed in order to adequately detect and diagnosis cardiac disease. For instance, a cardiac rest session and a cardiac stress session are required in the prior art that consume at minimum 30 minutes per session. Further, the patient must be allowed to rest for at least four hours in between cardiac stress/rest sessions. Taking in to account preparation and analysis time, the entire non-gated SPECT imaging session could consume well over six hours in total. Therefore, it would be advantageous to provide a system capable of accurately detecting infarct areas and ischemic areas of the heart without the need for two separate scan sessions performed in conjunction and without the intermediary rest period in between. The present invention offers such advantageous capability.
Further, prior art camera systems employing SPECT imaging, in non-gated perfusion studies, do not offer an advantageous method for detecting false positive determinations of an infarct area or an ischemic area of the heart. This is the case because other effects, such as attenuation of the radiation signal or statistical variations of the radiation distribution may cream artifacts within the image system that mimic diseased areas of an image. It is difficult to accurately and efficiently determine whether particular regions of images from these non-gated SPECT systems are actually an infarct or ischemic area or rather simply an artifact as a result of one of the above effects. For example, a resultant image from a male heart often contains artifacts (false positives) in the inferior heart area as a result of radiation attenuation of the diaphragm, which varies due to the diaphragm size. Also, a resultant image from a female heart often contains artifacts (false positives) in the anterior lateral to anterior septal areas of the heart as a result of radiation attenuation from the breasts, which may vary due to breast size. In these cases for both males and females, it is desirable to be able to detect and correct for these false positives. It would be advantageous to provide an efficient system for quantitatively testing false positive ischemic and infarct areas of the heart. The present invention offers such advantageous functionality.
Gated SPECT camera systems are similar in nature to the non-gated SPECT camera systems described above, however, the imaging of the object is gated at discrete intervals of time during the cardiac cycle for each discrete angle of rotation of the camera detector. Gated SPECT increases the sensitivity and specificity of diagnosis as compared to non-gated SPECT procedures because gated SPECT allows the observation of both characteristics of perfusion and function within cardiac physiology. For cardiac gated SPECT camera systems, the timing intervals are synchronized to different segments of the cardiac cycle. The heartbeat cycle contains locations indicating a systolic phase of the heart where the heart tissue is contracting to pump blood and a diastolic phase of the heart where the heart is expanding and filling with blood. By synchronizing the collection of imaging information from the heart (the object of interest in cardiac studies) at the diastolic and systolic phases of the cardiac cycle, the gated SPECT camera system can provide physicians with images of the heart during both contraction and expansion. This information is utilized in diagnosing heart disease, such as CAD. Gated SPECT camera systems can be utilized to image the heart at any timing segment within the heartbeat cycle (cardiac cycle). Typically in gated SPECT techniques, the image of the heart at the end of the diastolic phase (end-diastole) is recorded and studied and the image of the heart at the end of the systolic phase (end-systole) is recorded and studied.
According to gated SPECT studies, if a heart region is detected with a certain count density in the myocardium at maximum expansion (end-diastole) and this region does not show much increased count density at minimum expansion (end-systole), then the myocardium in the location of the defect may be ischemic or artifactual. If the count density remains constant over the time segments then the defect may represent an infarct or dead tissue. It would be advantageous to utilize the above principles in conjunction with wall movement data in a nuclear camera imaging system to provide quantitative information regarding the myocardium which can be used for diagnosis. The present invention offers such capability by providing specialized quantitative displays of the gated SPECT image data.
Prior art systems of gated SPECT nuclear camera systems have focused primarily on qualitative studies over quantitative studies. To this extent, images generated at end-diastole and end-systole have been presented to the diagnosing physician without any meaningful quantitative analysis of the structures or movements of structures of the heart. This leaves determination and diagnosis of possible diseased areas of the heart (i.e. infarct or ischemic areas) to approximate and non quantitatively based judgments on the part of the physician. It would be advantageous to provide a gated SPECT nuclear camera imaging system that offered quantitative analysis and measurement display of the heart region or regions under review. This quantitative data could then more effectively aid a physician in diagnosing areas of CAD and accurately reproduce such findings. The present invention offers such advantageous quantitative information analysis and display capability.
Other prior art systems determined ejection fractions as an alternate avenue for CAD diagnosis and employ gated SPECT camera systems qualitatively to determine the ejection fraction. The ejection fraction is the percent of total blood in the heart cavity that is actually ejected from the heart during contraction and expansion. Gated SPECT techniques are utilized to image the heart during contraction (systole) and during expansion (diastole) to determine the heart volumes at these periods which can be used in diagnosing heart disease to determine the ejection fraction. The ejection fraction is determined as a ratio between the difference of the volume of the heart at diastole and systole over the volume of the heart at diastole. A low ejection fraction may indicate an infarct or ischemic area.
The above prior art methods of determining the ejection fraction are limited because the determination method of the systolic and diastolic volumes is not accurate and the determination more often than not is the result of approximation and qualitative judgment based on qualitative information presented to the physician. Since the volume determinations are not quantitative, the ejection ratios determined are not quantitative and not readily reproduced lending various contradictory diagnosis for a given condition. Aside from the qualitative nature of the image data, physical limitations in the camera resolutions and partial volume effects severely degrade the accuracy and reproducibility of the determination of these two volumes. It would be advantageous to provide quantitative method for determining cardiac disease using gated SPECT techniques over the above prior art design. The present invention provides such capability.
Additionally, prior art nuclear camera systems collect data from the camera detector using two data parsing passes. The first parsing pass examines each byte or word of data that is detected by the camera detector and is used to construct a temporary histogram for a particular heart beat, this is called a beat histogram. After the first parsing processing is complete, a second processes sums the beat histogram data with the overall or total sum histogram that represents histogram data for all imaged beats for a given projection angle and for a given gated segment. If the newly collected beat histogram represents data from a heartbeat that is to be rejected, then the beat histogram data will be erased and therefore ignored. This prior art process requires a great deal of time and processing power because, essentially, the input data from the detector must be completely parsed twice before it is incorporated into the summation histogram. Further, if a particular heartbeat is to be ignored, it is wasteful of processing power and inefficient to construct the beat histogram. Therefore, what is needed is a method of determining bad beats without inefficient construction of the beat histogram. Further, what is needed is a way to implement the data acquisition processing of the nuclear camera system that can eliminate the double parsing required if data from a bad heartbeat is detected and avoid constructing a beat histogram that is never used. The present invention provides such capability. Further, the present invention also offers the capability, upon detection of a bad current heartbeat, of skipping the data representative of a just previously imaged heart beat.
Accordingly, it is an object of the present invention to provide a nuclear medicine imaging system for aiding in the diagnosis of cardiac disease using gated SPECT techniques. It is an object of the present invention to provide a nuclear medicine imaging system for aiding in the diagnosis of cardiac disease without requiring both a stress imaging session and a rest imaging session in conjunction. Further, it is an object of the present invention to provide a gated SPECT imaging system wherein wall motion and wall perfusion can be quantitatively computed and rendered with respect to various images of the heart at various gated segments of the cardiac cycle. It is also an object of the present invention to provide a nuclear imaging camera system capable of effectively and efficiently detecting false positives for infarct and ischemic areas of the heart. It is yet another object of the present invention to provide a gated SPECT system for providing a functional image that simultaneously displays both wall movement and wall thickening information. It is also an object of the present invention to provide an effective display system allowing efficient location, display and comparison of image frames of the multitude of image frames that are made available from the spatial slices and temporal segments of reconstructed volumes resultant from a gated SPECT study. It is an object of the present invention to provide an efficient data acquisition procedure of a camera system that has the ability to skip bad beat data events without constructing a beat histogram. These and various other objects not specifically mentioned above will become evident upon further review of the discussions of the present invention to follow.